A glow discharge starter is usually connected across or in parallel with an arc discharge lamp and contains a pair of electrodes. At least one of the electrodes comprises a bimetallic element which, when heated as a result of the glow discharge, bends towards the other electrode. When contact is made, the glow discharge ceases causing the bimetallic element to cool and withdraw from the contacted electrode. When contact is broken, a voltage pulse induced by the induction of the ballast, appears across the opposed electrodes of the lamp thereby initiating an arc discharge within the lamp. If the lamp ignition does not occur after the first voltage pulse, the glow discharge sequence is repeated until lamp ignition occurs.
An example of a glow discharge starter of the aforementioned type is described in the book "Light Sources" by Elenbaas, Philips Technical Library, pages 102-103. Other examples of glow discharge starters are shown and described in U.S. Pat. Nos. 2,321,910 and 2,285,805.
The glow discharge starter is designed such that the contacts close at a voltage chosen between the maximum lamp voltage and the minimum supply voltage (i.e., closure voltage). The contacts of the starter must also remain open at voltages less than the maximum lamp voltage (i.e., non-reclosure voltage). For 220 volt line specifications, a starter typically has a maximum closure voltage of 180 volts and a minimum non-reclosure voltage of 140 volts. It is important that the electrical parameters of the glow discharge starter remain within this range throughout the life of the starter. Some conventional glow discharge starters intended for high line voltage applications fail in less than 6000 cycles.
To make gases break down at a particular voltage and discharge extinguish at a slightly lower voltage, various methods have been employed in the past. It is known to use zinc in high voltage starters. In one commercially available starter, the zinc is plated on the bimetallic element at a thickness of from about 0.001 to 0.0005 inch. Such plating keeps the closure and non-reclosure voltages within a preestablished specification range for a maximum of approximately 3000 cycles. At the contact points, the zinc is rapidly depleted which exposes nickel-containing parts (e.g., bimetallic element, post and counter electrode) to the discharge. The exposed nickel is spattered over the internal surfaces of the glow discharge starter changing the work function. As a result, the glow discharge starter fails to maintain the closure and non-reclosure voltages within the preestablished limits past 3000 cycles. More specifically, the non-reclosure voltage has been found to drop to about 130 volts while the reclosure voltage increases to about 200 volts. U.S. Pat. No. 2,740,861 is another example of a glow discharge starter using a coating of zinc.
Silver has been used in the past as a contact material in glow discharge starters. During the seasoning process (i.e., aging) as well as during the entire life of the starter, the silver from the contacts is spattered over portions of the metal surfaces. The solid silver serves as a source of the work function reducing material during the life of the starter. However, since nickel-containing parts are still exposed to the electrical discharge, the electrical characteristics of the starter tend to change. As a result, these silver-containing starters resort to the use of complex gas mixtures to stabilize the characteristics of the glow discharge starters during life. Fill gas mixtures consisting of combinations of, for example, hydrogen, helium, neon, argon, and krypton 85 have been used. Hydrogen and helium can be absorbed by the starter envelope, getter material or internal metal parts which results in a change in the gas composition and consequently a change in the electrical characteristics of the glow discharge starter during life.